area rule

These photos show the narrowing of the YF-102A fuselage
near the wings as dictated by the area rule compared to the fuselage
of the YF-102. It enabled the aircraft to become supersonic. Credit:
NASA

A rule in aircraft design discovered by NASA aerodynamicist
Richard Whitcomb in 1950. It states that, in order to produce the least
amount of drag when approaching supersonic
flight, the cross-sectional area of an aircraft body should be consistent
throughout the aircraft's length. To compensate for the place on an aircraft
where the wings are attached to the fuselage, the fuselage needs to be made
narrower so that the cross-section remains the same. This is why aircraft
that are designed to fly around the speed
of sound have a pinched fuselage where the wings are attached to the
body.